To provide a method of working a cylinder such as a damper cylinder, which is excellent in yield, also in workability and by which the inside diameter can be set to arbitrary dimensions.

In a method of working the cylinder by which thickness in both end parts is thickened and thickness in the intermediate part is thinned, a means for solving problems is constituted of: a stage where the inside diameter of one end of a tube stock 1 is made into a small diameter by narrowing down the one end; a stage where the end part of a core bar 2 is supported with the small-diameter end part of the tube stock 1 after inserting the core bar into the tube stock 1; and a stage where the tube stock 1 and the core bar 2 are rotated and spinning is performed while pinching and pressing the tube stock 1 with the core bar 2 and one or a plurality of rollers 3 while making the tube stock 1 and the core bar 2 rotate and also moving one or the plurality of rollers 3 which are in press-contact with the outer periphery of the tube stock 1 in the axial direction. Both the end parts are thickened and the intermediate part is thinned by making the movement locus of the rollers 3 approach the side of the core bar 2 in the range corresponding to the intermediate part of the tube stock 1 and also, by providing a large-diameter part 2c having an arbitrary length in the portion of the core bar 2 corresponding to the intermediate part where is formed into thin, the inside diameter in the intermediate part corresponding to this large-diameter part 2c is formed in large.